Data transmission system

ABSTRACT

A data transmission system in which normal transmission can be performed irrespectively of the inserting orientation of a connector is provided. A transmitting device transmits to a receiving device a differential transmission signal including polarity decision data for deciding the polarity of the connector. Based on the polarity decision data included in the differential transmission signal transmitted from the transmitting device, the receiving device decides whether the polarity of the connector has been reversed or not. When it is decided that the polarity has not been reversed, the receiving device reads data from the differential transmission signal. When it is decided that the polarity has been reversed, the polarity of the differential transmission signal is reversed for data reading.

FIELD OF THE INVENTION

The present invention relates generally to data transmission systemsand, more specifically, to a data transmission system for differentialtransmission by using a transmission signal having a polarity.

BACKGROUND OF THE INVENTION

In conventional encoded data transmission technologies, various types ofcables are used for achieving noise-resistant transmission. Of suchvarious cables, coaxial cables capable of preventing noise by shieldinga signal line are generally used. However, the coaxial cables haveproblems of requiring high shielding cost and heavy cable weight.

One technology known to solve the above problems is a differentialtransmission technology using a twist pair cable. In this technology,with a differential receiver removing in-phase components, it ispossible to achieve a high anti-noise capability. Furthermore, when atwist pair cable is used, shielding is not required, unlike the case ofthe coaxial cable. Thus, cost reduction and light-weight design can beadvantageously achieved.

The twist pair cable is used for transmission with two lines. Therefore,a signal inherently has a polarity. For this reason, as for aconventional connector used for the twist pair cable, a correctinserting orientation and the polarity of the cable to be connected arepredetermined. Therefore, when a user erroneously connects the connectorwith its inserting orientation (the polarity of the connector) reversedor when a cable of reversed connection is used, a signal with itspolarity reversed is received by a device. In this case, data cannot becorrectly received.

In one case, a connection is achieved so as not to make a connector withan incorrect polarity of the cable (so as not to make a cable and aconnector reversed in polarity between a transmission side and areception side). In this case, however, the cable and the connector haveto be made in consideration of the polarities at the transmission sideand the reception side (so that the polarity at the transmission sidealways coincides with that at the reception side). Therefore, itrequires time and cost to manufacture the cable and the connector.

Therefore, an object of the present invention is to provide a datatransmission system in which normal transmission can be performedregardless of the polarity of the cable.

SUMMARY OF THE INVENTION

To achieve the object as mentioned above, the present invention hasfeatures as described below.

The present invention has the following features to achieve the aboveobject. That is, the present invention is directed to a system in whichdata is transmitted between a transmitting device and a receiving deviceby transmitting a differential signal by using two transmission lineshaving a polarity. The transmitting device generates a differentialtransmission signal including a polarity decision signal whose signallevel is constant for a length including a predetermined number ofpieces of symbol data, and sends the differential transmission signal tothe transmission lines. On the other hand, the receiving device includesa connector section, a timing correcting section, a polarity decidingsection, and a signal processing section. The connector section isremovably connected to the two transmission lines for receiving thedifferential transmission signal transmitted from the transmittingdevice when being connected to the transmission lines. The timingcorrecting section corrects a detection timing (e.g., when the detectiontiming for detecting a signal level is at a symbol position in thedifferential transmission signal) when a predetermined process performedon the differential transmission signal received by the connectorsection is successively incorrect for symbol data whose number is largerthan the predetermined number. The polarity deciding section detects thepolarity decision signal included in the differential transmissionsignal received by the connector section and, based on a signal level ofthe polarity decision signal, decides whether a connecting relationshipof the connector section with the transmission lines has a positivepolarity or a reversed polarity. When it is decided that the connectingrelationship of the connector section with the transmission lines hasthe positive polarity, the signal processing section handles thedifferential transmission signal received by the connector section as asignal having a normal polarity. In this case, the signal processingsection performs the predetermined process. When it is decided that theconnecting relationship of the connector section with the transmissionlines has the reversed polarity, the signal processing section handlesthe differential transmission signal as a signal having a reversedpolarity and performs the predetermined process. According to the above,based on the polarity decision signal, it is decided whether to handlethe differential transmission signal as its polarity has been reversedor as it is. Therefore, even when the connector section is connectedreversely in polarity and consequently the polarity of the receiveddifferential transmission signal is reversed, the occurrence of reversalis accurately decided, and the reversed differential transmission signalcan be reliably corrected. With the above, normal data transmission canbe performed irrespectively of the inserting orientation of theconnector and the polarity of the connected cable.

Note that, whether the detection timing is correct or incorrect cannotbe determined with the polarity decision signal because its signal levelis constant. That is, even when symbol data included in the polaritydecision signal is tried to be detected, it might not be possible toaccurately perform a correcting process and accurately detect a symbolincluded in a signal to be received thereafter. However, according tothe above, the detection timing is corrected when it is successivelyincorrect for pieces of symbol data whose number is larger than thepredetermined number. Thus, correction of the detection timing is notperformed as to the polarity decision signal. Therefore, the symbol dataincluded in the differential transmission signal can be reliablydetected. That is, it is possible to reliably establish synchronization.

Note that, in the above data transmission system, the differentialtransmission signal may further include a sync-establishing signal whichis transmitted prior to the polarity decision signal and is generated soas to have a signal waveform having a predetermined period. At thistime, based on the signal waveform of the sync-establishing signalincluded in the differential transmission signal received by theconnector section, the timing correcting section determines thedetection timing for detection of a signal level of a signal receivedafter the sync-establishing signal.

Also, in the above transmission system, the differential transmissionsignal may further include a transmission data signal which istransmitted after the polarity decision signal and is generated so thata symbol position of data to be transmitted comes at a vertex of awaveform. At this time, the timing correcting section determines whetherthe detection timing is incorrect or not based on whether a signaldetecting position for detection of the signal level in the differentialtransmission signal in the detection timing is located at the vertex ofthe signal waveform of the differential transmission signal.

Furthermore, the signal processing section may include a normalprocessing section and a polarity-reversed processing section. Thenormal processing section performs a first process on the differentialtransmission signal received by the connector section when it is decidedthat the connecting relationship of the connector section with thetransmission lines has a positive polarity. The polarity-reversedprocessing section performs a second process on the differentialtransmission signal received by the connector section when it is decidedthat the connecting relationship of the connector section with thetransmission lines has a reversed polarity. Also, the normal processingsection and the polarity-reversed processing section perform the firstand second processes, respectively, so that same process results arededuced for the same differential transmission signal being transmittedon the transmission lines.

According to the above, depending on whether the polarity of theconnector or the cable has been reversed or not, either of theprocessing sections for performing two different processes (the normalprocessing section and the polarity-reversed processing section) isused. When the connector is connected with a positive polarity, thedifferential transmission signal is processed by the normal processingsection. On the other hand, when the connector is connected with areversed polarity, the differential transmission is processed by thepolarity-reversed processing section so that the same process resultscan be obtained as those obtained when a signal with its polarityreversed is processed by the normal processing section. With the above,by appropriately using either of the two processing sections, thedifferential transmission signal can be easily corrected even when thedifferential transmission signal has been reversed.

Furthermore, the signal processing section may include a polarityreversing section and a normal processing section. The polarityreversing section reverses the polarity of the differential transmissionsignal received by the connector section when it is decided that theconnecting relationship of the connector section with the transmissionlines has a reversed polarity. The normal processing section performs,when it is decided that the relationship of the connector section withthe transmission lines has a positive polarity, the predeterminedprocess on the differential transmission signal received by theconnector section, and performs, when it is decided that therelationship of the connector section with the transmission lines has areversed polarity, the predetermined process on the differentialtransmission signal whose polarity has been reversed by the polarityreversing section.

According to the above, the polarity of the received differentialtransmission signal is reversed only when the polarity of the connectorhas been reversed. Also, the predetermined process is performed on thedifferential transmission signal reversed or non-reversed in polaritybased on the decision result of whether the polarity has been reversed.Therefore, only one circuit for conversion to decoded data is required.Therefore, the circuit size can be reduced.

Note that, in the above data transmission system, in the differentialtransmission signal, data of not less than 1 bit may be a signal to eachsignal level as one symbol.

Note that the present invention can be provided as a signal processingcircuit receiving a differential signal transmitted by using twotransmission lines having a polarity via a connector removably connectedto the transmission lines and performing a predetermined process. Thesignal processing circuit includes an input terminal, a timingcorrecting section, a polarity deciding section, and a signal processingsection. The input terminal is to input from the connector adifferential transmission signal including a polarity decision signalwhose signal level is constant for a length including a predeterminednumber of pieces of symbol data. The timing correcting section correctsa detection timing when the detecting timing for detecting a signallevel at a symbol position in the differential transmission signal whenthe predetermined process is performed on the differential transmissionsignal input from the input terminal is successively incorrect forsymbol data whose number is larger than the predetermined number. Thepolarity deciding section detects the polarity decision signal includedin the differential transmission signal input from the input terminaland, based on a signal level of the polarity decision signal, decideswhether a connecting relationship of the connector with the transmissionlines has a positive polarity or a reversed polarity. When it is decidedthat the connecting relationship of the connector with the transmissionlines has the positive polarity, the signal processing section handlesthe differential transmission signal received by the connector as asignal having a normal polarity and performs the predetermined process.However, when it is decided that the connecting relationship of theconnector with the transmission lines has the reversed polarity, thesignal processing section handles the differential transmission signalas a signal having a reversed polarity and performs the predeterminedprocess.

Furthermore, the present invention may be provided as a datatransmission method carried out in the above data transmission system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the configuration of a datatransmission system according to a first embodiment of the presentinvention.

FIG. 2 is a block diagram illustrating a hardware structure of a device1 illustrated in FIG. 1.

FIG. 3 is a block diagram illustrating a detailed structure of adifferential transmission signal processing section 12 illustrated inFIG. 2.

FIG. 4 is an illustration for describing an input/output relationship inan A/D converter illustrated in FIG. 3.

FIG. 5 is an illustration for describing details of the operation of atiming reproducing section in accordance with an embodiment of thepresent invention.

FIG. 6 is a flowchart illustrating a flow of a process performed by atransmitting device in the first embodiment.

FIG. 7 is a part of a flowchart illustrating a flow of a processperformed by a receiving device in the first embodiment.

FIG. 8 is a part of the flowchart illustrating the flow of the processperformed by the receiving device in the first embodiment.

FIG. 9 is an illustration schematically showing values indicated by dataoutput from a symbol data extracting section illustrated in FIG. 3.

FIG. 10 is an illustration schematically showing values indicated bydata output from the symbol data extracting section illustrated in FIG.3 when the polarity of a differential transmission signal has beenreversed.

FIG. 11 is an illustration for describing a scheme of calculatingthreshold values in accordance with an embodiment of the presentinvention.

FIG. 12 is a block diagram illustrating a detailed structure of a datadeciding section illustrated in FIG. 3.

FIG. 13 is an illustration showing a conversion correspondence in twodecoding circuits included in the data deciding section illustrated inFIG. 12.

FIG. 14 is a flowchart illustrating a flow of a process performed by areceiving device in a second embodiment of the present invention.

FIG. 15 is a block diagram illustrating a detailed structure of a datadeciding section in the second embodiment.

FIG. 16 is an illustration schematically showing a polarity reversingprocess in a polarity reversing circuit illustrated in FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram illustrating the configuration of a datatransmission system according to a first embodiment of the presentinvention. As illustrated in FIG. 1, the data transmission systemconstructs a ring-type network with devices 1 through 5. Therefore, inthe present embodiment, a device (e.g., 1) receives data from oneadjacent device (e.g., 2) and transmits the data to the other adjacentdevice (e.g., 3), thereby achieving data transmission in a ring shape onthe network. Here, the devices (e.g., 1-5) for data transmission areconnected via a twist pair cable having a polarity. Also, datatransmission among the devices is performed through differentialtransmission using a differential signal. Note that, in anotherembodiment, the configuration of the network is not limited to a ringshape, but may take any shape as long as differential transmission isperformed among the devices in the system via a transmission line havinga polarity.

FIG. 2 is a block diagram illustrating a hardware structure of thedevice 1 illustrated in FIG. 1. Note that, although FIG. 2 illustratesthe structure of the device 1, all devices illustrated in FIG. 1 (thedevices 1 through 5) have the structure illustrated in FIG. 2. In FIG.2, the device 1 includes a connector 11, a differential transmissionsignal processing section 12, an upper-layer data processing section 13,and a CPU 14. The connector 11 is removably connected to another devicevia a twist pair cable not shown for transmission/reception of adifferential transmission signal. Here, the differential transmissionsignal is a differential signal to be transmitted and received in datatransmission among the devices. The differential transmission signalincludes transmission data to be transmitted among the devices, and alsoincludes, when power of the network is turned on, and polarity decisiondata for deciding the polarity of the connector. The polarity decisiondata is data for deciding the polarity of the connector by deciding thepolarity of the differential transmission signal. With the use of thepolarity decision data, the data transmission system makes it possibleto perform normal data transmission irrespectively of the insertingorientation of the connector. The differential transmission signal willbe described in detail further below.

The differential transmission signal processing section 12 decodes thedifferential transmission signal received via the connector 11 through apredetermined decoding process which will be described further below.Here, the data decoded by the differential transmission signalprocessing section 12 is referred to as decoded data. Also, thedifferential transmission signal processing section 12 outputs datainput from the upper-layer data processing section 13 as a differentialtransmission signal. The output differential transmission signal istransmitted via the connector to another device. Also, when power of thenetwork is turned on, the differential transmission signal processingsignal 12 adds the polarity decision data to the data input from theupper-layer data processing section 13, and then outputs the resultantsignal as a differential transmission signal. The upper-layer dataprocessing section 13 performs predetermined format conversion betweendata to be processed in the CPU 14 and data to be processed in thedifferential transmission signal processing section 12.

FIG. 3 is a block diagram illustrating a detailed structure of thedifferential transmission signal processing section 12 illustrated inFIG. 2. The differential transmission signal processing section 12includes a transmission processing section 201, a D/A converter 202, alow-pass filter (LPF) 203, a driver 204, a receiver 205, an A/Dconverter (denoted as “A/D” in FIG. 3) 206, a digital filter 207, atiming reproducing section 208, a symbol data extracting section 209, adata deciding section 210, a polarity deciding section 211, and atraining processing section 212.

When power of the network is turned on, the transmission processingsection 201 outputs initialization data (sync-establishing data, whichwill be described further below, the above-mentioned polarity decisiondata, and training data, which will be described further below), andthen outputs data input from the upper-layer data processing section 13.The data output from the transmission processing section 201 isD/A-converted by the D/A converter 202, and is then transmitted via theLPF 203 and the driver 204 to another device.

Here, in the present embodiment, it is assumed that 2-bit octaltransmission per symbol is performed by using a mapping in which any oneof upper quaternary symbols and any one of lower quaternary symbols arealternately selected. That is, it is assumed that the D/A converter 202converts digital data of 2 bits into analog data. Note that, in thedifferential transmission signal, data of 2 bits is assigned to eachsignal level as one symbol. Also, the differential transmission signalis generated so as to include symbol data at every predetermined timeinterval of T.

In the present embodiment, the differential transmission signal isstructured of a sync-establishing signal, a polarity decision signal, atraining signal, and a transmission data signal. The sync-establishingsignal is a signal including the sync-establishing data in order to makethe receiving device identify the polarity decision data and startdeciding the polarity of the connector. The polarity decision signal isa signal including the polarity decision data for deciding the polarityof the connector. The training signal is a signal used in a trainingprocess, which will be described further below. The transmission datasignal is a signal including data to be transmitted.

Also, the polarity decision signal has a predetermined waveform pattern,and is constant in signal level for a length including a predeterminednumber of pieces of symbol data. In the present embodiment, the polaritydecision signal has a pattern in which the same value continues forthree symbols (refer to FIG. 9).

Furthermore, in the present embodiment, the differential transmissionsignal is generated so that, as for portions other than the polaritydecision signal, the symbol is located at a peak of the waveform. Thatis, the waveform of the differential transmission signal (excluding theportion of the polarity decision signal) has a waveform pattern in whichthe waveform has a peak at every predetermined time interval of T, andthe position of the peak is the position of the symbol. As for thedifferential transmission signal transmitted from another device, eachdevice illustrated in FIG. 1 reads the signal level at the position ofthe symbol, and then converts the read signal level into digital data,thereby reading the transmission data.

In FIG. 3, the differential transmission signal transmitted from anotherdevice is received via the connector 11 by the receiver 205. Thedifferential transmission signal received by the receiver 205 isA/D-converted by the A/D converter 206. The A/D-converted differentialtransmission signal is supplied to the digital filter 207 and the timingreproducing section 208. Here, the A/D converter 206 converts the inputanalog data to digital data of plural bits. In the present embodiment,the A/D converter 206 converts the input analog data to digital data of10 bits.

FIG. 4 is an illustration for describing an input-output relationship inthe A/D converter 206 illustrated in FIG. 3. In FIG. 4, a curverepresents analog data input to the A/D converter 206. The A/D converter206 samples the analog data at every predetermined sampling interval oft (t is sufficiently shorter than the above predetermined time intervalof T), and converts the magnitude of the signal level to digital data of10 bits. Note that the digital data of 10 bits does not represent thecontents of data included in the differential transmission signal. Notethat, in the present embodiment, sampling at every interval of t that isshorter than the above predetermined time interval of T and convertingto digital data of 10 bits are performed in order to accuratelyreproduce multi-valued symbols of the differential transmission signal.In the present embodiment, the waveform of the differential transmissionsignal is converted to digital data of 10 bits, and then the digitaldata of 10 bits is further converted to digital data of 2 bits. It isthe digital data of 2 bits that represents the contents of the dataincluded in the differential transmission signal.

As described above, the differential transmission signal converted bythe A/D converter 206 to the digital data of 10 bits is supplied to thedigital filter 207 and the timing reproducing section 208. The digitalfilter 207 eliminates a high-frequency noise component from the inputdigital data of 10 bits. The timing reproducing section 208 decides thetiming of detecting symbol data in the digital data of 10 bits. Thedetection timing is the timing of extracting meaningful data (symboldata) from the digital data of 10 bits. The time interval of thedetection timing is an interval in which symbol data is included in thedifferential transmission signal, that is, the above predetermined timeinterval of T. As for the digital data A/D-converted at every samplinginterval of t in the A/D converter 206, not all of the output digitaldata necessarily represent meaningful symbol data. For this reason, thetiming of extracting the symbol data from the digital data output fromthe A/D converter 206 is decided by the timing reproducing section 208.

Hereinafter, details of the operation of the timing reproducing section208 are described. FIG. 5 is an illustration for describing details ofthe operation of the timing reproducing section 208. Points shown inFIG. 5 represent digital data of 10 bits sequentially input from the A/Dconverter 206 at every time interval of t. The timing reproducingsection 208 extracts data in accordance with the detection timingdetermined at that moment from the data input from the A/D converter206. The timing reproducing section 208 compares the data extracted inthe detection timing (data at a time ta), data input immediately beforethe detection timing (data at a time tb), and data input immediatelyafter the detection timing (data at a time tc). Here, as illustrated inFIG. 5( a), when the value of the data extracted in the detection timingis the largest (or the smallest), it is determined that the detectiontiming is correct. On the other hand, as illustrated in FIG. 5( b), whenthe value of the data extracted in the detection timing is between thedata input immediately before the detection timing and the data inputimmediately after the detection timing, it is determined that thedetection timing is incorrect. The above-stated determination isperformed at every detection timing.

Furthermore, as for the data input from the A/D converter 206, when thedetection timing is incorrect successively for a predetermined number ofpieces of symbol data, the timing reproducing section 208 corrects thedetection timing. Here, the predetermined number is determined by thenumber of successive pieces of symbol data whose values are equal toeach other in the polarity decision signal. In the present embodiment,the number of successive pieces of symbol data whose values are equal toeach other in the polarity decision signal is three. Therefore, thepredetermined number can be any as long as it is larger than three.

With this, at the time of reading symbol data included in the polaritydecision signal, a case can be prevented where it is determined that thedetection timing is incorrect for the predetermined number of successivepieces of symbol data. Here, if it is determined that the detectiontiming is incorrect, the polarity decision signal is constant in signallevel (signal level values at the above times ta, tb, and tc are equalto each other), and therefore correction cannot be made accurately.Therefore, if it is determined that the detection timing is incorrect,the detection timing cannot be made accurately, and therefore the signalthereafter may not be correctly read. For this reason, in the presentembodiment, the length of a portion of the polarity decision signal inwhich the signal level is constant is made shorter than the length of aportion in which the detection timing is determined to be corrected.This ensures reliable and accurate signal reading.

Note that the timing reproducing section 208 corrects the detectiontiming so that the detection timing becomes a timing of detecting theposition of the vertex of the waveform. A specific correcting measure tobe taken can be any. For example, in a case where detection results areas illustrated in FIG. 5( b), when a maximum value should be detected incorrect detection timing, the time tb is thought to be the closest tothe correction detection timing. Therefore, in this case, correction ismade so that a time point after the time T has elapsed from the time tbis the next detection timing.

In FIG. 3, the symbol data extracting section 209 extracts data from thedigital data input from the digital filter 207 in accordance with thesymbol timing determined by the timing reproducing section 208. That is,the timing reproducing section 208 sends an instruction to the symboldata extracting section 209 for extracting data in the determinedtiming. In response to this instruction, the symbol data extractingsection 209 extracts symbol data from the digital data of 10 bits inputfrom the digital filter 207. The extracted symbol data is supplied tothe data deciding section 210, the polarity deciding section 211, andthe training processing section 212.

The polarity deciding section 211 decides, based on the polaritydecision data transmitted prior to the transmission data, whether thepolarity of the connector, that is, the connection relationship betweenthe twist pair cable and the connector, has a positive polarity or areversed polarity. The decision result is supplied to the data decidingsection 210. The training processing section 212 determines, based onthe training data transmitted prior to the transmission data, thresholdvalues for converting the signal level of the differential transmissionsignal to multi-valued (here, octal) digital data. The determinedthreshold values are supplied to the data deciding section 210. The datadeciding section 210 performs a predetermined data deciding process,that is, the above predetermined decoding process, for converting thetransmission data included in the differential transmission signal todata that can be processed by the upper-layer data processing section13. Also, when the polarity is determined at the polarity decidingsection 211, the data deciding section 210 changes the decoding resultin accordance with the decision result obtained by the polarity decidingsection 211. The decoded data, that is, the decoded transmission data,is output to the upper-layer data processing section 13.

Next, the operation in the present data transmission system when powerof the network is turned on is described. Note that, in the following, adevice transmitting a differential transmission signal is taken as atransmitting device, and a device receiving the differentialtransmission signal is taken as a receiving device. Here, in the presentembodiment, when power of the network is turned on, the polarity of theconnector is decided and the polarity of the signal is reversed inaccordance with the decision result, thereby performing normal datatransmission irrespectively of the inserting orientation of theconnector. Note that, since the system according to the presentembodiment forms a ring-shaped network, if the connector is removed orinserted or if a new device is added to the network, it is alwaysrequired to turn off the power of the network. Furthermore, as describedabove, in the present embodiment, the polarity of the connector isdecided when power is turned on. Therefore, in the present datatransmission system, a decision can be always made when it is requiredto decide the connector's polarity (especially when the connector isinserted/removed, etc.).

FIG. 6 is a flowchart illustrating a flow of a process performed by thetransmitting device in the present embodiment. Also, FIG. 7 and FIG. 8are a flowchart illustrating a flow of a process performed by thereceiving device in the present embodiment. The operation of each of thedevices illustrated in FIG. 6, FIG. 7, and FIG. 8 is started uponturning-on of the power of the network. Here, in the ring-shapednetwork, data transmission cannot be performed unless power is turned onin the respective devices forming the network. Therefore, in the presentembodiment, the state in which power of the network is turned on meansthat power of all of the devices included in the data transmissionsystem is turned on. Note that a scheme of controlling the turning-on ofpower of all of the devices forming the network can be any. For example,a device for controlling a power supply of each device forming thenetwork may be placed, and this device turns on power of all of thedevices.

When power of the network is turned on, the transmitting devicetransmits a sync-establishing signal including sync-establishing data(step S101). Here, the sync-establishing data is data for the receivingdevice to identify the polarity decision data and start a process ofdeciding the polarity of the connector. Also, in the present embodiment,the sync-establishing data is used as initialization data to betransmitted for starting an initializing process performed in eachdevice. Note that the sync-establishing data has a predetermined givenpattern. Specifically, in the process of step S101, the transmissionprocessing section 201 of the transmitting device generates thesync-establishing data when power of the transmitting device is turnedon. The generated sync-establishing data is transmitted via the D/Aconverter 202, the LPF 203, and the driver 204 to the receiving device.In this manner, the process of step S101 is performed.

Next, the transmitting device decides whether or not a predeterminedtime has elapsed (step S102). Here, the predetermined time is set inadvance so as to be equal to or longer than a time required for thereceiving device, which is a transmission destination of thesync-establishing data, to complete establishment of synchronization.When it is decided in step S102 that the predetermined time has notelapsed, the transmitting device repeats the process of step S101. Onthe other hand, when it is decided that the predetermined time haselapsed, the transmitting device transmits a polarity decision signalincluding polarity decision data, which follows the sync-establishingsignal (step S103). Specifically, the transmission processing section201 of the transmitting device transmits the polarity decision datahaving the predetermined given pattern in a manner similar to that ofthe above sync-establishing data. The length of the polarity decisiondata is predetermined.

After transmitting the polarity decision signal by the predeterminedlength, the transmitting device transmits a training signal, whichfollows the polarity decision signal (step S104). The training signal isused for setting threshold values for deciding multi-valued (here,octal) digital data from the signal level of the differentialtransmission signal. The pattern and length of the training signal arepredetermined. Note that the scheme of transmitting the training signalis similar to that of the above sync-establishing data.

Next, the transmitting device transmits a transmission data signalincluding transmission data, which follows the training signal (stepS105). Specifically, after transmitting the training signal having thegiven pattern, the transmission processing section 201 of thetransmitting device transmits the transmission data input from theupper-layer data processing section 13 as a differential transmissionsignal. The transmission scheme is similar to that of the abovesync-establishing data. With the transmission of the transmission datato be transmitted being completed, the transmission processing section201 of the transmitting device ends the process illustrated in FIG. 6.As such, through the process illustrated in FIG. 6, the differentialtransmission signal structured of, in order of transmission, thesync-establishing signal, the polarity decision signal, the trainingsignal, and the transmission data signal, is transmitted to thereceiving device.

Next, the process performed by the receiving device illustrated in FIG.7 is described. In the receiving device, the sync-establishing signal isfirst received from the transmitting device as the differentialtransmission signal. Therefore, when power of the network is turned on,the receiving device first receives the sync-establishing signal (stepS201). The sync-establishing signal is supplied via the receiver 205 tothe A/D converter 206. The input sync-establishing signal isA/D-converted by the A/D converter 206, and the A/D-converted digitaldata is then supplied to the digital filter 207 and the timingreproducing section 208.

Next, based on the sync-establishing signal received in step S201, thereceiving device establishes synchronization (step S202). The process ofestablishing synchronization is performed by the timing reproducingsection 208. That is, the timing reproducing section 208 determines thetiming of detecting the level of the signal transmitted after thesync-establishing signal (detection timing). Determination of thedetection timing is made by performing a process of correcting theabove-described detection timing to make the detection timing corrected.The timing reproducing section 208 indicates the detection timingdetermined in step S202 to the symbol data extracting section 209.

Next, the receiving device decides whether the process of establishingsynchronization has been completed (step S203). This process isperformed by the timing reproducing section 208, and is designed so thatthe decision in step S203 has been completed before the predeterminedtime in the above step S102 has elapsed. When it is determined that theprocess of establishing synchronization has not been completed, thereceiving device again performs the process of step S201.

On the other hand, when it is determined in step S203 that the processof establishing synchronization has been completed, the receiving devicereceives the polarity decision signal transmitted in step S103 from thetransmitting device (step S204). The polarity decision signal issupplied via the receiver 205 of the receiving device to the A/Dconverter 206. The input polarity decision signal is A/D-converted bythe A/D converter 206. Furthermore, from the digital data of theA/D-converted polarity decision signal, the symbol data is extracted bythe symbol data extracting section 209.

Next after step S204, the receiving device detects the polarity of theconnector (step S205). This process is performed by the polaritydeciding section 211. Specifically, the polarity deciding section 211 ofthe receiving device receives the polarity decision signal output fromthe symbol data extracting section 209 and, based on the receivedpolarity decision signal, detects whether or not the polarity of thedifferential transmission signal (polarity decision signal) has beenreversed. As such, depending on whether or not the polarity of thedifferential transmission signal has been reversed, it can be decidedwhether or not the polarity of the connector has been reversed. Here, itis assumed in the present embodiment that, when the value of thepolarity decision data is at a maximum level of the differentialtransmission signal, that is, when the value of the polarity decisiondata is 1024, it is decided that the polarity of the differentialtransmission has not been reversed.

Details of a polarity deciding scheme are described below. FIG. 9 is anillustration schematically showing values indicated by data output fromthe symbol data extracting section 209 illustrated in FIG. 3. In FIG. 9,the vertical axis represents the magnitude of digital values, which areoutput values of the symbol data extracting section 209, and thehorizontal axis represents time for output. Furthermore, a digital valueoutput from the symbol data extracting section 209 is a value of 10bits, and its magnitude represents the level of the differentialtransmission signal. That is, the output value of the symbol dataextracting section 209 can be any one from 1 to 1024, numericallyrepresenting the level of the differential transmission signal.Furthermore, an interval between points on a line illustrated in FIG. 9is equal to the above-described time interval of T. The line representsthe waveform of the differential transmission signal by using discretenumerical values.

Here, the state from times t0 to t1 is such that the abovesync-establishing data is being output. That is, the pattern in whichoutput values a and b are alternately output from the times t0 to t1 isa pattern indicating the sync-establishing data. Such a pattern is setin advance. Note that, from the times t0 to t1, the processes of stepsS201 through S203 are performed.

Next, the state from times t1 to t2 is such that the polarity decisiondata included in the polarity decision signal is being output. That is,the pattern in which output values of 1024 are successively output fromthe times t1 to t2 is a pattern of the polarity decision data. As withthe sync-establishing data, the pattern of the polarity decision data ispredetermined. Here, the polarity decision data has a pattern in whichthe same values are successively observed for three symbols. Note that,from the times t1 to t2, the processes of steps S204 and S205 areperformed. As such, in the present embodiment, the polarity of theconnector can be decided by using the amplitude level of thedifferential transmission signal after the pattern of thesync-establishing data.

FIG. 10 is an illustration schematically showing values indicated bydata output from the symbol data extracting section 209 illustrated inFIG. 3 in a case where the polarity of the differential transmissionsignal has been reversed. FIG. 10 shows the output values of the symboldata extracting section 209 when data similar to that illustrated inFIG. 9 is received and when the connecting orientation of the connectoris reversed from that illustrated in FIG. 9. As such, in a case wherethe polarity of the connector has been reversed, the differentialtransmission signal received by the receiving device is a reversedsignal compared with a case where the polarity of the connector ispositive. That is, the differential transmission signal received by thereceiving device has a polarity, and this polarity is varied dependingon the polarity of the connector. As illustrated in FIG. 10, if theorientation of the connector has been reversed, the value to be read asthe polarity decision data is 1. Therefore, in this case, it is decidedthat the polarity of the differential transmission signal has beenreversed. As such, whether or not the polarity of the differentialtransmission signal has been reversed can be decided with the value ofthe polarity decision data. Thus, with the value of the polaritydecision data, the polarity deciding section 211 decides whether or notthe polarity of the differential transmission signal has been reversed,that is, whether or not the connecting orientation of the connector hasbeen reversed. Furthermore, the polarity deciding section 211 outputs tothe data deciding section 210 a signal polarity flag indicative ofwhether or not the polarity has been reversed.

Returning to the description of FIG. 7, next after step S205, thereceiving device receives the training signal transmitted in step S104from the transmitting device (step S206). Note that the processesperformed on the training signal by the receiver 205 through the symboldata extracting section 209 are similar to those performed on thepolarity decision signal. Next, the receiving device uses the trainingsignal received in step S206 to perform a training process (step S207).This process is performed by the training processing section 212.Details of the training process are described below.

In FIG. 9, the state from times t2 to t3 is such that the trainingsignal is being output. The training signal has a predetermined pattern.This pattern is determined so that the value of octal digital data takesall values (eight values) in a predetermined order. The trainingprocessing section 212 stores the order of the octal digital valuesindicated by the pattern. In step S207, the training processing section212 stores the value of the digital data of 10 bits input from thesymbol data extracting section 209 in relation to the stored octaldigital values.

Returning to the description of FIG. 7, next after step S207, thetraining processing section 212 decides whether or not training has beencompleted. Since the length of the training signal is predetermined, thenumber of times of performing the process of step S207 is alsopredetermined. If the process of step S207 has been performed for thepredetermined number of times, the training processing section 212decides that training has been completed. When it is decided thattraining has not yet been completed, the process of step S206 isperformed.

On the other hand, when it is decided that training has been completed,the training processing section 212 calculates threshold values (stepS209). The threshold values are values for converting the signal levelsof the differential transmission signal to octal digital data.

FIG. 11 is an illustration for describing one example of a scheme ofcalculating threshold values. In FIG. 11, the range of values (1 through1024) the signal level can take is divided into eight levels from LevelA to Level H according to the magnitude of the signal level. Level Athrough D correspond to values of four types an octal digital value cantake (“00”, “01”, “10”, and “11”), respectively. Similarly, Level Ethrough H correspond to values of four types a quaternary digital valuecan take (“00”, “01”, “10”, and “11”), respectively. The trainingprocessing section 212 sets the threshold values based on the value ofthe digital data of 10 bits input in step S207 from the symbol dataextracting section 209 and the octal digital values stored in advance.Specifically, a first threshold value for distinguishing between Level Aand Level B is set in the following manner. That is, as the firstthreshold value, the training processing section 212 takes an averagevalue (median value) between a minimum value of digital data of 10 bitsinput as having a value indicative of Level A and a maximum value ofdigital data of 10 bits input as having a value indicative of Level B.Second through seventh threshold values for distinguishing other levelsare set in a similar manner as that of the above first threshold value.The first through seventh threshold values set in the above-describedmanner are supplied to the data deciding section 210.

In FIG. 8, next after step S209, the receiving device receives thetransmission data signal (step S210). Note that, in FIG. 9, the stateafter the time t3 is such that the transmission data signal is beingoutput. The data after the time t3 is decoded by the data decidingsection 210. Furthermore, the processes performed on the transmissiondata signal by the receiver 205 through the symbol data extractingsection 209 are similar to those performed on the polarity decisionsignal.

Next, the receiving device decides whether or not the polarity of thedifferential transmission signal (transmission data signal) has beenreversed (step S211). This process is performed by the data decidingsection 210. Specifically, the data deciding section 210 decides whetheror not the polarity of the differential transmission signal has beenreversed based on the signal polarity flag input from the polaritydeciding section 211. When it is decided in step S211 that the polarityof the differential transmission signal has been reversed, the receivingdevice performs a decoding process for a case where the polarity hasbeen reversed (step S212). On the other hand, when it is decided thatthe polarity of the differential transmission signal has not beenreversed, the receiving device performs a normal decoding process (stepS213). Here, the processes of step S212 and step S213 are preformed bythe data deciding section 210 of the receiving device. Details of theoperation performed by the data deciding section 210 are describedbelow.

FIG. 12 is a block diagram illustrating a detailed structure of the datadeciding section 210 illustrated in FIG. 3. In FIG. 12, the datadeciding section 210 includes a selecting circuit 2101, a normaldecoding circuit 2102, and a polarity-reversed decoding circuit 2103. Instep S206, in accordance with the indication of the signal polarity flaginput from the polarity deciding section 211, the selecting circuit 2101selects one of the normal decoding circuit 2102 and thepolarity-reversed decoding circuit 2103 to which the output signal fromthe A/D converter 206 is to be supplied. Upon reception of a signalpolarity flag indicating that the polarity of the differentialtransmission signal has been reversed, the selecting circuit 2101outputs the output signal from the A/D converter 206 to thepolarity-reversed decoding circuit 2103. In step S207, thepolarity-reversed decoding circuit 2103 receives the output signal fromthe A/D converter 206, and then decodes the transmission data, which isthe received output signal. On the other hand, upon reception of asignal polarity flag indicating that the polarity of the differentialtransmission signal has not been reversed, the selecting circuit 2101outputs the output signal from the A/D converter 206 to the normaldecoding circuit 2102. In step S208, the normal decoding circuit 2102receives the output signal from the A/D converter 206, and then decodesthe transmission data, which is the received output signal.

FIG. 13 is an illustration showing a conversion correspondence in thetwo decoding circuits included in the data deciding section 210illustrated in FIG. 12. FIG. 13( a) is a table indicative of acorrespondence in the normal decoding circuit 2102, and FIG. 13( b) is atable indicative of a correspondence in the polarity-reversed decodingcircuit 2103. Here, it is assumed in the present embodiment that eachdecoding circuit converts the digital data of 10 bits output from theA/D converter 206 to digital data of 2 bits. Therefore, in eachconversion table illustrated in FIG. 13, the digital values of 10 bitsoutput from the A/D converter 206 (values each indicative of a level ofthe differential transmission signal) are divided into eight levels, andare respectively related to numerical values of 2 bits indicative ofdecoded data. Note that the threshold values of eight levels shown inFIG. 13 are the above-described first through seventh threshold values.Here, the two conversion tables shown in FIG. 13 are generated so thatthe digital values of 10 bits corresponding to the decoded data to beconverted appear on these two conversion tables with the polarity inreverse to each other. That is, the normal decoding circuit 2102 and thepolarity-reversed decoding circuit 2103 are designed so that thepolarity of the differential transmission signal corresponding to thedecoded data to be output is in reverse to each other. With these twodecoding circuits being designed in the above manner, the decoded dataobtained through decoding by using the normal decoding circuit when thepolarity of the signal has not been reversed can be equal to the decodeddata obtained through decoding by using the polarity-reversed decodingcircuit when the polarity of the signal has been reversed.

Upon completion of the above-described process of step S212 or S213, thereceiving device ends the processing. Note that the decoded dataobtained through decoding in step S212 or S213 is forwarded via theupper-layer data processing section 13 to the CPU 14.

Note in the above first embodiment that the above sync-establishing datais set in advance so that the waveform of the differential transmissionsignal added with the synch-establishing data includes a waveformpattern in which the waveform is the same irrespectively of whether thepolarity of the connector is positive or reversed. That is, asillustrated in FIG. 9, the synch-establishing data includes the patternin which a and b are alternately output as the output value. Such apattern is the same when the connector is connected so as to have apositive polarity and when the connector is connected so as to have areversed polarity. Therefore, by detecting the sync-establishing databased on such a pattern, the receiving device can reliably detect thesync-establishing data.

Next, a second embodiment is described. Note that the first embodimentand the second embodiment are different in the structure of the datadeciding section 210 and the decoding process performed by the datadeciding section 210. Therefore, only these differences are describedbelow, and processes similar to those in the first embodiment are notdescribed herein.

FIG. 14 is a flowchart showing a flow of a process performed by thereceiving device in the second embodiment. Here, the present embodimentis different from the first embodiment in the processes of steps S301and S302. Therefore, the processes of steps S201 through S211 are notdescribed herein. In step S211, upon decision that the polarity of thedifferential transmission signal has been reversed, the receiving devicereverses the polarity of the differential transmission signal (stepS301), and then performs the process of step S302. On the other hand,upon decision that the polarity of the differential transmission signalhas not been reversed, the receiving device does not perform the processof step S301 but performs the process of step S302. Details of theoperation performed by the data deciding section 210 are describedbelow.

FIG. 15 is a block diagram illustrating a detailed structure of the datadeciding section 210 in the second embodiment. In FIG. 15, the datadeciding section 210 includes a selecting circuit 2104, a polarityreversing circuit 2105, and a normal decoding circuit 2106. In stepS211, in accordance with the indication of the signal polarity flaginput from the polarity deciding section 211, the selecting circuit 2104selects either one of the polarity reversing circuit 2105 and the normaldecoding circuit 2106 to which the output signal from the A/D converter206 is to be supplied. Upon reception of a signal polarity flagindicating that the polarity of the differential transmission signal hasbeen reversed, the selecting circuit 2104 outputs the output signal fromthe A/D converter 206 to the polarity reversing circuit 2105. Thepolarity reversing circuit 2105 reverses the polarity of thedifferential transmission signal indicating the input transmission data(step S301).

FIG. 16 is an illustration schematically showing a polarity reversingprocess in the polarity reversing circuit 2105 illustrated in FIG. 15.The polarity reversing circuit 2105 converts the input value so that theinput value has a line-symmetric relationship with the median value(512) of the values (1 from 1024) that can be taken as that input value.For example, as illustrated in FIG. 16, when the input value from theA/D converter to the polarity reversing circuit 2105 is 700 (point D),the polarity reversing circuit 2105 converts the input value to 324(point D′) for output to the normal decoding circuit 2106. With thisconverting process, the polarity reversing circuit 2105 can reverse thepolarity of the differential transmission signal.

On the other hand, upon reception of a signal polarity flag indicatingthat the polarity of the differential transmission signal has not beenreversed, the selecting circuit 2104 outputs the output signal from theA/D converter 206 to the normal decoding circuit 2106. The normaldecoding circuit 2106 receives an output from the A/D converter 206 orthe polarity reversing circuit 2105, and decodes the receivedtransmission data (step S302). The normal decoding circuit 2106 performsa process similar to that performed by the normal decoding circuit 2102in the first embodiment. With this, the digital data of 10 bitsgenerated in the A/D converter 206 is converted to digital data of 2bits (decoded data).

Upon completion of the above-described process of step S302, thereceiving device ends the processing. Note that the decoded data decodedin step S302 is forwarded via the upper-layer data processing section 13to the CPU 14.

As described above, in the above first and second embodiments, thepolarity of the connector is decided by using the polarity decision dataand, based on the decision result, the polarity of the differentialtransmission signal is reversed or non-reversed, thereby performingcorrect data transmission irrespectively of the polarity of theconnector.

Note that, in the above first and second embodiments, description hasbeen made to a case as an example in which a decoding process isperformed on the differential transmission signal received by thereceiving device. Here, in another embodiment, a process performed onthe differential transmission signal is not limited to the above. Theprocess performed on the differential transmission signal may be any aslong as it changes a manner of handling the differential transmissionsignal received by the connector 11 to a manner in which the signal ishandled as a signal with a normal polarity or to a manner in which thesignal is handled as a signal with a reversed polarity, depending on acase where it is decided that the connecting relationship of theconnector 11 with the twist pair cable has a positive polarity or a casewhere it is decided that the connecting relationship has a reversedpolarity.

Also, in the above first and second embodiments, the polarity of theconnector is decided by using the polarity decision data when power isturned on. Therefore, the decision result has to be stored when thepolarity of the connector is decided at the time of turning on thepower. Although not shown in the above embodiments, the present datatransmission system is provided with a storage section for storing thepolarity of the connector decided at the time of turning on the power.Note that, in another embodiment, the polarity decision data may bealways added prior to the transmission data, and the polarity of theconnector may be decided every time the transmission data istransmitted. Furthermore, the polarity decision data may be alwaysadded, and the receiving device may decide the polarity of the connectoronly when required. For example, if the receiving device has a functionof detecting removal/insertion of the connector, the polarity of theconnector may be decided every time removal/insertion of the connectoris detected.

Furthermore, the above first and second embodiments are embodiments inwhich the differential transmission signal is A/D converted and is thenreversed. Here, in another embodiment, the differential transmissionsignal may be reversed by using an analog circuit before beingA/D-converted.

Furthermore, other than the above first and second embodiments, thepolarity may be reversed after the decoded data is generated.Specifically, a converting circuit may be provided for converting thedigital data of 2 bits generated as the decoded data, and the digitalvalue may be converted by the converting circuit in accordance with thesignal polarity flag. In this case, the converting circuit is designedso that the decoded data is converted to decoded data generated in acase where the polarity of the differential transmission signal has beenreversed.

Furthermore, in the above first and second embodiments, the pattern inwhich a and b are alternately output as an output value is included,thereby enabling the receiving device to reliably detect thesync-establishing data. On the other hand, in another embodiment, thesync-establishing data may be any as long as it has a predeterminedpattern. Also, in this case, the receiving device preferably stores thepredetermined pattern and a pattern for a case where the polarity of thedifferential transmission signal including this pattern has beenreversed. And the receiving device detects a differential transmissionsignal corresponding to either of the two patterns stored in advance tostart deciding the polarity. Also with this scheme, the receiving devicecan reliably detect the synch-establishing data.

Furthermore, in the above first and second embodiments, a signal levelvalue in each symbol is read, and the value is then converted to adigital value of 2 bits. Here, a scheme of conversion to a digital valueof 2 bits is not limited to the above. For example, a difference betweenone symbol and the previous symbol may be read and then be converted toa digital value of 2 bits. In this case, the data deciding sectionperforms a data deciding process (conversion to digital data of 2 bits)by using a differential value between a value of the input signal level(which corresponds to the digital data of 10 bits in the aboveembodiments) and a level value of a signal input in the previous timing.

As described above, the data transmission system of the presentinvention can be used for the purpose of performing normal transmissionirrespectively of the inserting orientation of the connector.

1. A system in which data is transmitted between a transmitting deviceand a receiving device by transmitting a differential signal by usingtwo transmission lines having a polarity, the transmitting device beingoperable to generate a differential transmission signal including apolarity decision signal whose signal level is constant for a lengthincluding a predetermined number of pieces of symbol data and sendingthe differential transmission signal to the transmission lines, thereceiving device including: a connector section removably connected tothe two transmission lines for receiving the differential transmissionsignal transmitted from the transmitting device when being connected tothe transmission lines; a timing correcting section operable to correcta detection timing when the detection timing for detecting a signallevel at a symbol position in the differential transmission signal, issuccessively incorrect for a plurality of symbol data whose number islarger than the number of pieces of symbol data representing thepolarity decision signal; a polarity deciding section operable to detectthe polarity decision signal included in the differential transmissionsignal received by the connector section and to decide, based on asignal level of the polarity decision signal, whether a connectingrelationship of the connector section with the transmission lines has apositive polarity or a reversed polarity; and a signal processingsection operable to handle the differential transmission signal receivedby the connector section as a signal having a normal polarity andperform a predetermined process when it is decided that the connectingrelationship of the connector section with the transmission lines hasthe positive polarity, and operable to handle the differentialtransmission signal received by the connector section as a signal havinga reversed polarity and perform the predetermined process when it isdecided that the connecting relationship of the connector section withthe transmission lines has the reversed polarity.
 2. The datatransmission system according to claim 1, wherein the differentialtransmission signal further includes a sync-establishing signal which istransmitted prior to the polarity decision signal and is generated so asto have a signal waveform having a predetermined period, and based onthe signal waveform of the sync-establishing signal included in thedifferential transmission signal received by the connector section, thetiming correcting section determines the detection timing for detectionof a signal level of a signal received after the sync-establishingsignal.
 3. The transmission system according to claim 1, wherein thedifferential transmission signal further includes a transmission datasignal which is transmitted after the polarity decision signal and isgenerated so that a symbol position of data to be transmitted is at avertex of a waveform, and the timing correcting section is operable todetermine whether the detection timing is incorrect or not based onwhether a signal detecting position for detection of the signal level inthe differential transmission signal in the detection timing is locatedat the vertex the signal waveform of the differential transmissionsignal.
 4. The data transmission system according to claim 1, whereinthe signal processing section includes: a normal processing sectionoperable to perform a first process on the differential transmissionsignal received by the connector section when it is decided that theconnecting relationship of the connector section with the transmissionlines has a positive polarity; and a polarity-reversed processingsection operable to perform a second process on the differentialtransmission signal received by the connector section when it is decidedthat the connecting relationship of the connector section with thetransmission lines has a reversed polarity, and the normal processingsection and the polarity-reversed processing section operable to performthe first and second processes, respectively, so that same processresults are deduced for the same differential transmission signal beingtransmitted on the transmission lines.
 5. The data transmission systemaccording to claim 1, wherein the signal processing section includes: apolarity reversing section operable to reverse the polarity of thedifferential transmission signal received by the connector section whenit is decided that the connecting relationship of the connector sectionwith the transmission lines has a reversed polarity; and a normalprocessing section operable to perform when it is decided that therelationship of the connector section with the transmission lines has apositive polarity, the predetermined process on the differentialtransmission signal received by the connector section, and operable toperform, when it is decided that the relationship of the connectorsection with the transmission lines has a reversed polarity, thepredetermined process on the differential transmission signal whosepolarity has been reversed by the polarity reversing section.
 6. Thedata transmission system according to claim 1, wherein in thedifferential transmission signal, data of not less than 1 bit isassigned to each signal level as one symbol.
 7. A signal processingcircuit operable to receive a differential signal transmitted by usingtwo transmission lines having a polarity via a connector removablyconnected to the transmission lines and to perform a predeterminedprocess, comprising: an input terminal arranged to input from theconnector a differential transmission signal including a polaritydecision signal whose signal level is constant for a length including apredetermined number of pieces of symbol data; a timing correctingsection operable to correct a detection timing when the detecting timingfor detecting a signal level at a symbol position in the differentialtransmission signal, is successively incorrect for a plurality of symboldata whose number is larger than the number of pieces of symbol datarepresenting the polarity decision signal; a polarity deciding sectionoperable to detect the polarity decision signal included in thedifferential transmission signal input from the input terminal andoperable to decide, based on a signal level of the polarity decisionsignal, whether a connecting relationship of the connector with thetransmission lines has a positive polarity or a reversed polarity; and asignal processing section operable to handle the differentialtransmission signal received by the connector as a signal having anormal polarity and perform a predetermined process when it is decidedthat the connecting relationship of the connector with the transmissionlines has the positive polarity, and operable to handle the differentialtransmission signal received by the connector as a signal having areversed polarity and perform a predetermined process when it is decidedthat the connecting relationship of the connector with the transmissionlines has the reversed polarity.
 8. The signal processing circuitaccording to claim 7, wherein the differential transmission signalfurther includes a sync-establishing signal which is transmitted priorto the polarity decision signal and is generated so as to have a signalwaveform having a predetermined period, and based on the signal waveformof the sync-establishing signal included in the differentialtransmission signal inputted from the input terminal, the timingcorrecting section is operable to decide the detection timing fordetection of a signal level of a signal received after thesync-establishing signal.
 9. The signal processing circuit according toclaim 7, wherein the differential transmission signal further includes atransmission data signal which is transmitted after the polaritydecision signal and is generated so that a symbol position of data to betransmitted comes at a vertex of a waveform, and the timing correctingsection is operable to decide whether the detection timing is incorrector not based on whether a signal detecting position for detection of thesignal level in the differential transmission signal in the detectiontiming is located at the vertex of the signal waveform of thedifferential transmission signal.
 10. The signal processing circuitaccording to claim 7, wherein the signal processing section includes: anormal processing section operable to perform a first process on thedifferential transmission signal received by the connector when it isdecided that the connecting relationship of the connector with thetransmission lines has a positive polarity; and a polarity-reversedprocessing section operable to perform a second process on thedifferential transmission signal received by the connector when it isdecided that the connecting relationship of the connector with thetransmission lines has a reversed polarity, and the normal processingsection and the polarity-reversed processing section operable to performthe first and second processes, respectively, so that same processresults are deduced for the same differential transmission signaltransmitted on the transmission lines.
 11. The signal processing circuitaccording to claim 7, wherein the signal processing section includes: apolarity reversing section operable to reverse the polarity of thedifferential transmission signal received by the connector when it isdecided that the relationship of the connector with the transmissionlines has a reversed polarity; and a normal processing section operableto perform, when it is decided that the relationship of the connectorwith the transmission lines has a positive polarity, the predeterminedprocess on the differential transmission signal received by theconnector, and operable to perform, when it is decided that therelationship of the connector with the transmission lines has a reversedpolarity, the predetermined process on the differential transmissionsignal whose polarity has been reversed by the polarity reversingsection.
 12. The signal processing circuit according to claim 7, whereinin the differential transmission signal, data of not less than 1 bit isassigned to each signal level as one symbol.
 13. A method oftransmitting data between a transmitting device and a receiving deviceby transmitting a differential signal by using two transmission lineshaving a polarity, the method comprising, in the transmitting device,generating a differential transmission signal including a polaritydecision signal whose signal level is constant for a length including apredetermined number of pieces of symbol data and sending thedifferential transmission signal to the transmission lines, and in thereceiving device: receiving the differential transmission signaltransmitted from the transmitting device via a connector removablyinserted to the two transmission lines; correcting a detection timingwhen the detection timing for detection of a signal level at a symbolposition from the differential transmission signal, via the connector,is successively incorrect for a plurality of symbol data whose number islarger than the number of pieces of symbol data representing thepolarity decision signal; detecting the polarity decision signalincluded in the differential transmission signal received by theconnector and, based on a signal level of the polarity decision signal,deciding whether a connecting relationship of the connector with thetransmission lines has a positive polarity or a reversed polarity; andhandling the differential transmission signal received by the connectoras a signal having a normal polarity and performing a predeterminedprocess when it is decided that the connecting relationship of theconnector with the transmission lines has the positive polarity, andhandling the differential transmission signal received by the connectoras a signal having a reversed polarity and performing the predeterminedprocess when it is decided that the connecting relationship of theconnector with the transmission lines has the reversed polarity.